1 //===- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop -------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This pass transforms loops that contain branches on loop-invariant conditions 11 // to multiple loops. For example, it turns the left into the right code: 12 // 13 // for (...) if (lic) 14 // A for (...) 15 // if (lic) A; B; C 16 // B else 17 // C for (...) 18 // A; C 19 // 20 // This can increase the size of the code exponentially (doubling it every time 21 // a loop is unswitched) so we only unswitch if the resultant code will be 22 // smaller than a threshold. 23 // 24 // This pass expects LICM to be run before it to hoist invariant conditions out 25 // of the loop, to make the unswitching opportunity obvious. 26 // 27 //===----------------------------------------------------------------------===// 28 29 #include "llvm/ADT/DenseMap.h" 30 #include "llvm/ADT/SmallPtrSet.h" 31 #include "llvm/ADT/SmallPtrSet.h" 32 #include "llvm/ADT/SmallVector.h" 33 #include "llvm/ADT/Statistic.h" 34 #include "llvm/Analysis/AssumptionCache.h" 35 #include "llvm/Analysis/CodeMetrics.h" 36 #include "llvm/Analysis/DivergenceAnalysis.h" 37 #include "llvm/Analysis/InstructionSimplify.h" 38 #include "llvm/Analysis/LoopInfo.h" 39 #include "llvm/Analysis/LoopPass.h" 40 #include "llvm/Analysis/ScalarEvolution.h" 41 #include "llvm/Analysis/TargetTransformInfo.h" 42 #include "llvm/Transforms/Utils/Local.h" 43 #include "llvm/IR/Attributes.h" 44 #include "llvm/IR/BasicBlock.h" 45 #include "llvm/IR/CallSite.h" 46 #include "llvm/IR/Constant.h" 47 #include "llvm/IR/Constants.h" 48 #include "llvm/IR/DerivedTypes.h" 49 #include "llvm/IR/Dominators.h" 50 #include "llvm/IR/Function.h" 51 #include "llvm/IR/IRBuilder.h" 52 #include "llvm/IR/InstrTypes.h" 53 #include "llvm/IR/Instruction.h" 54 #include "llvm/IR/Instructions.h" 55 #include "llvm/IR/IntrinsicInst.h" 56 #include "llvm/IR/Intrinsics.h" 57 #include "llvm/IR/Module.h" 58 #include "llvm/IR/Type.h" 59 #include "llvm/IR/User.h" 60 #include "llvm/IR/Value.h" 61 #include "llvm/IR/ValueHandle.h" 62 #include "llvm/Pass.h" 63 #include "llvm/Support/Casting.h" 64 #include "llvm/Support/CommandLine.h" 65 #include "llvm/Support/Debug.h" 66 #include "llvm/Support/raw_ostream.h" 67 #include "llvm/Transforms/Scalar.h" 68 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 69 #include "llvm/Transforms/Utils/Cloning.h" 70 #include "llvm/Transforms/Utils/LoopUtils.h" 71 #include "llvm/Transforms/Utils/ValueMapper.h" 72 #include <algorithm> 73 #include <cassert> 74 #include <map> 75 #include <set> 76 #include <tuple> 77 #include <utility> 78 #include <vector> 79 80 using namespace llvm; 81 82 #define DEBUG_TYPE "loop-unswitch" 83 84 STATISTIC(NumBranches, "Number of branches unswitched"); 85 STATISTIC(NumSwitches, "Number of switches unswitched"); 86 STATISTIC(NumGuards, "Number of guards unswitched"); 87 STATISTIC(NumSelects , "Number of selects unswitched"); 88 STATISTIC(NumTrivial , "Number of unswitches that are trivial"); 89 STATISTIC(NumSimplify, "Number of simplifications of unswitched code"); 90 STATISTIC(TotalInsts, "Total number of instructions analyzed"); 91 92 // The specific value of 100 here was chosen based only on intuition and a 93 // few specific examples. 94 static cl::opt<unsigned> 95 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), 96 cl::init(100), cl::Hidden); 97 98 namespace { 99 100 class LUAnalysisCache { 101 using UnswitchedValsMap = 102 DenseMap<const SwitchInst *, SmallPtrSet<const Value *, 8>>; 103 using UnswitchedValsIt = UnswitchedValsMap::iterator; 104 105 struct LoopProperties { 106 unsigned CanBeUnswitchedCount; 107 unsigned WasUnswitchedCount; 108 unsigned SizeEstimation; 109 UnswitchedValsMap UnswitchedVals; 110 }; 111 112 // Here we use std::map instead of DenseMap, since we need to keep valid 113 // LoopProperties pointer for current loop for better performance. 114 using LoopPropsMap = std::map<const Loop *, LoopProperties>; 115 using LoopPropsMapIt = LoopPropsMap::iterator; 116 117 LoopPropsMap LoopsProperties; 118 UnswitchedValsMap *CurLoopInstructions = nullptr; 119 LoopProperties *CurrentLoopProperties = nullptr; 120 121 // A loop unswitching with an estimated cost above this threshold 122 // is not performed. MaxSize is turned into unswitching quota for 123 // the current loop, and reduced correspondingly, though note that 124 // the quota is returned by releaseMemory() when the loop has been 125 // processed, so that MaxSize will return to its previous 126 // value. So in most cases MaxSize will equal the Threshold flag 127 // when a new loop is processed. An exception to that is that 128 // MaxSize will have a smaller value while processing nested loops 129 // that were introduced due to loop unswitching of an outer loop. 130 // 131 // FIXME: The way that MaxSize works is subtle and depends on the 132 // pass manager processing loops and calling releaseMemory() in a 133 // specific order. It would be good to find a more straightforward 134 // way of doing what MaxSize does. 135 unsigned MaxSize; 136 137 public: 138 LUAnalysisCache() : MaxSize(Threshold) {} 139 140 // Analyze loop. Check its size, calculate is it possible to unswitch 141 // it. Returns true if we can unswitch this loop. 142 bool countLoop(const Loop *L, const TargetTransformInfo &TTI, 143 AssumptionCache *AC); 144 145 // Clean all data related to given loop. 146 void forgetLoop(const Loop *L); 147 148 // Mark case value as unswitched. 149 // Since SI instruction can be partly unswitched, in order to avoid 150 // extra unswitching in cloned loops keep track all unswitched values. 151 void setUnswitched(const SwitchInst *SI, const Value *V); 152 153 // Check was this case value unswitched before or not. 154 bool isUnswitched(const SwitchInst *SI, const Value *V); 155 156 // Returns true if another unswitching could be done within the cost 157 // threshold. 158 bool CostAllowsUnswitching(); 159 160 // Clone all loop-unswitch related loop properties. 161 // Redistribute unswitching quotas. 162 // Note, that new loop data is stored inside the VMap. 163 void cloneData(const Loop *NewLoop, const Loop *OldLoop, 164 const ValueToValueMapTy &VMap); 165 }; 166 167 class LoopUnswitch : public LoopPass { 168 LoopInfo *LI; // Loop information 169 LPPassManager *LPM; 170 AssumptionCache *AC; 171 172 // Used to check if second loop needs processing after 173 // RewriteLoopBodyWithConditionConstant rewrites first loop. 174 std::vector<Loop*> LoopProcessWorklist; 175 176 LUAnalysisCache BranchesInfo; 177 178 bool OptimizeForSize; 179 bool redoLoop = false; 180 181 Loop *currentLoop = nullptr; 182 DominatorTree *DT = nullptr; 183 BasicBlock *loopHeader = nullptr; 184 BasicBlock *loopPreheader = nullptr; 185 186 bool SanitizeMemory; 187 LoopSafetyInfo SafetyInfo; 188 189 // LoopBlocks contains all of the basic blocks of the loop, including the 190 // preheader of the loop, the body of the loop, and the exit blocks of the 191 // loop, in that order. 192 std::vector<BasicBlock*> LoopBlocks; 193 // NewBlocks contained cloned copy of basic blocks from LoopBlocks. 194 std::vector<BasicBlock*> NewBlocks; 195 196 bool hasBranchDivergence; 197 198 public: 199 static char ID; // Pass ID, replacement for typeid 200 201 explicit LoopUnswitch(bool Os = false, bool hasBranchDivergence = false) 202 : LoopPass(ID), OptimizeForSize(Os), 203 hasBranchDivergence(hasBranchDivergence) { 204 initializeLoopUnswitchPass(*PassRegistry::getPassRegistry()); 205 } 206 207 bool runOnLoop(Loop *L, LPPassManager &LPM) override; 208 bool processCurrentLoop(); 209 bool isUnreachableDueToPreviousUnswitching(BasicBlock *); 210 211 /// This transformation requires natural loop information & requires that 212 /// loop preheaders be inserted into the CFG. 213 /// 214 void getAnalysisUsage(AnalysisUsage &AU) const override { 215 AU.addRequired<AssumptionCacheTracker>(); 216 AU.addRequired<TargetTransformInfoWrapperPass>(); 217 if (hasBranchDivergence) 218 AU.addRequired<DivergenceAnalysis>(); 219 getLoopAnalysisUsage(AU); 220 } 221 222 private: 223 void releaseMemory() override { 224 BranchesInfo.forgetLoop(currentLoop); 225 } 226 227 void initLoopData() { 228 loopHeader = currentLoop->getHeader(); 229 loopPreheader = currentLoop->getLoopPreheader(); 230 } 231 232 /// Split all of the edges from inside the loop to their exit blocks. 233 /// Update the appropriate Phi nodes as we do so. 234 void SplitExitEdges(Loop *L, 235 const SmallVectorImpl<BasicBlock *> &ExitBlocks); 236 237 bool TryTrivialLoopUnswitch(bool &Changed); 238 239 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val, 240 TerminatorInst *TI = nullptr); 241 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 242 BasicBlock *ExitBlock, TerminatorInst *TI); 243 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L, 244 TerminatorInst *TI); 245 246 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 247 Constant *Val, bool isEqual); 248 249 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 250 BasicBlock *TrueDest, 251 BasicBlock *FalseDest, 252 BranchInst *OldBranch, 253 TerminatorInst *TI); 254 255 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); 256 257 /// Given that the Invariant is not equal to Val. Simplify instructions 258 /// in the loop. 259 Value *SimplifyInstructionWithNotEqual(Instruction *Inst, Value *Invariant, 260 Constant *Val); 261 }; 262 263 } // end anonymous namespace 264 265 // Analyze loop. Check its size, calculate is it possible to unswitch 266 // it. Returns true if we can unswitch this loop. 267 bool LUAnalysisCache::countLoop(const Loop *L, const TargetTransformInfo &TTI, 268 AssumptionCache *AC) { 269 LoopPropsMapIt PropsIt; 270 bool Inserted; 271 std::tie(PropsIt, Inserted) = 272 LoopsProperties.insert(std::make_pair(L, LoopProperties())); 273 274 LoopProperties &Props = PropsIt->second; 275 276 if (Inserted) { 277 // New loop. 278 279 // Limit the number of instructions to avoid causing significant code 280 // expansion, and the number of basic blocks, to avoid loops with 281 // large numbers of branches which cause loop unswitching to go crazy. 282 // This is a very ad-hoc heuristic. 283 284 SmallPtrSet<const Value *, 32> EphValues; 285 CodeMetrics::collectEphemeralValues(L, AC, EphValues); 286 287 // FIXME: This is overly conservative because it does not take into 288 // consideration code simplification opportunities and code that can 289 // be shared by the resultant unswitched loops. 290 CodeMetrics Metrics; 291 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); I != E; 292 ++I) 293 Metrics.analyzeBasicBlock(*I, TTI, EphValues); 294 295 Props.SizeEstimation = Metrics.NumInsts; 296 Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation); 297 Props.WasUnswitchedCount = 0; 298 MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount; 299 300 if (Metrics.notDuplicatable) { 301 LLVM_DEBUG(dbgs() << "NOT unswitching loop %" << L->getHeader()->getName() 302 << ", contents cannot be " 303 << "duplicated!\n"); 304 return false; 305 } 306 } 307 308 // Be careful. This links are good only before new loop addition. 309 CurrentLoopProperties = &Props; 310 CurLoopInstructions = &Props.UnswitchedVals; 311 312 return true; 313 } 314 315 // Clean all data related to given loop. 316 void LUAnalysisCache::forgetLoop(const Loop *L) { 317 LoopPropsMapIt LIt = LoopsProperties.find(L); 318 319 if (LIt != LoopsProperties.end()) { 320 LoopProperties &Props = LIt->second; 321 MaxSize += (Props.CanBeUnswitchedCount + Props.WasUnswitchedCount) * 322 Props.SizeEstimation; 323 LoopsProperties.erase(LIt); 324 } 325 326 CurrentLoopProperties = nullptr; 327 CurLoopInstructions = nullptr; 328 } 329 330 // Mark case value as unswitched. 331 // Since SI instruction can be partly unswitched, in order to avoid 332 // extra unswitching in cloned loops keep track all unswitched values. 333 void LUAnalysisCache::setUnswitched(const SwitchInst *SI, const Value *V) { 334 (*CurLoopInstructions)[SI].insert(V); 335 } 336 337 // Check was this case value unswitched before or not. 338 bool LUAnalysisCache::isUnswitched(const SwitchInst *SI, const Value *V) { 339 return (*CurLoopInstructions)[SI].count(V); 340 } 341 342 bool LUAnalysisCache::CostAllowsUnswitching() { 343 return CurrentLoopProperties->CanBeUnswitchedCount > 0; 344 } 345 346 // Clone all loop-unswitch related loop properties. 347 // Redistribute unswitching quotas. 348 // Note, that new loop data is stored inside the VMap. 349 void LUAnalysisCache::cloneData(const Loop *NewLoop, const Loop *OldLoop, 350 const ValueToValueMapTy &VMap) { 351 LoopProperties &NewLoopProps = LoopsProperties[NewLoop]; 352 LoopProperties &OldLoopProps = *CurrentLoopProperties; 353 UnswitchedValsMap &Insts = OldLoopProps.UnswitchedVals; 354 355 // Reallocate "can-be-unswitched quota" 356 357 --OldLoopProps.CanBeUnswitchedCount; 358 ++OldLoopProps.WasUnswitchedCount; 359 NewLoopProps.WasUnswitchedCount = 0; 360 unsigned Quota = OldLoopProps.CanBeUnswitchedCount; 361 NewLoopProps.CanBeUnswitchedCount = Quota / 2; 362 OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2; 363 364 NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation; 365 366 // Clone unswitched values info: 367 // for new loop switches we clone info about values that was 368 // already unswitched and has redundant successors. 369 for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) { 370 const SwitchInst *OldInst = I->first; 371 Value *NewI = VMap.lookup(OldInst); 372 const SwitchInst *NewInst = cast_or_null<SwitchInst>(NewI); 373 assert(NewInst && "All instructions that are in SrcBB must be in VMap."); 374 375 NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst]; 376 } 377 } 378 379 char LoopUnswitch::ID = 0; 380 381 INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops", 382 false, false) 383 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 384 INITIALIZE_PASS_DEPENDENCY(LoopPass) 385 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 386 INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis) 387 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops", 388 false, false) 389 390 Pass *llvm::createLoopUnswitchPass(bool Os, bool hasBranchDivergence) { 391 return new LoopUnswitch(Os, hasBranchDivergence); 392 } 393 394 /// Operator chain lattice. 395 enum OperatorChain { 396 OC_OpChainNone, ///< There is no operator. 397 OC_OpChainOr, ///< There are only ORs. 398 OC_OpChainAnd, ///< There are only ANDs. 399 OC_OpChainMixed ///< There are ANDs and ORs. 400 }; 401 402 /// Cond is a condition that occurs in L. If it is invariant in the loop, or has 403 /// an invariant piece, return the invariant. Otherwise, return null. 404 // 405 /// NOTE: FindLIVLoopCondition will not return a partial LIV by walking up a 406 /// mixed operator chain, as we can not reliably find a value which will simplify 407 /// the operator chain. If the chain is AND-only or OR-only, we can use 0 or ~0 408 /// to simplify the chain. 409 /// 410 /// NOTE: In case a partial LIV and a mixed operator chain, we may be able to 411 /// simplify the condition itself to a loop variant condition, but at the 412 /// cost of creating an entirely new loop. 413 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed, 414 OperatorChain &ParentChain, 415 DenseMap<Value *, Value *> &Cache) { 416 auto CacheIt = Cache.find(Cond); 417 if (CacheIt != Cache.end()) 418 return CacheIt->second; 419 420 // We started analyze new instruction, increment scanned instructions counter. 421 ++TotalInsts; 422 423 // We can never unswitch on vector conditions. 424 if (Cond->getType()->isVectorTy()) 425 return nullptr; 426 427 // Constants should be folded, not unswitched on! 428 if (isa<Constant>(Cond)) return nullptr; 429 430 // TODO: Handle: br (VARIANT|INVARIANT). 431 432 // Hoist simple values out. 433 if (L->makeLoopInvariant(Cond, Changed)) { 434 Cache[Cond] = Cond; 435 return Cond; 436 } 437 438 // Walk up the operator chain to find partial invariant conditions. 439 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 440 if (BO->getOpcode() == Instruction::And || 441 BO->getOpcode() == Instruction::Or) { 442 // Given the previous operator, compute the current operator chain status. 443 OperatorChain NewChain; 444 switch (ParentChain) { 445 case OC_OpChainNone: 446 NewChain = BO->getOpcode() == Instruction::And ? OC_OpChainAnd : 447 OC_OpChainOr; 448 break; 449 case OC_OpChainOr: 450 NewChain = BO->getOpcode() == Instruction::Or ? OC_OpChainOr : 451 OC_OpChainMixed; 452 break; 453 case OC_OpChainAnd: 454 NewChain = BO->getOpcode() == Instruction::And ? OC_OpChainAnd : 455 OC_OpChainMixed; 456 break; 457 case OC_OpChainMixed: 458 NewChain = OC_OpChainMixed; 459 break; 460 } 461 462 // If we reach a Mixed state, we do not want to keep walking up as we can not 463 // reliably find a value that will simplify the chain. With this check, we 464 // will return null on the first sight of mixed chain and the caller will 465 // either backtrack to find partial LIV in other operand or return null. 466 if (NewChain != OC_OpChainMixed) { 467 // Update the current operator chain type before we search up the chain. 468 ParentChain = NewChain; 469 // If either the left or right side is invariant, we can unswitch on this, 470 // which will cause the branch to go away in one loop and the condition to 471 // simplify in the other one. 472 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed, 473 ParentChain, Cache)) { 474 Cache[Cond] = LHS; 475 return LHS; 476 } 477 // We did not manage to find a partial LIV in operand(0). Backtrack and try 478 // operand(1). 479 ParentChain = NewChain; 480 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed, 481 ParentChain, Cache)) { 482 Cache[Cond] = RHS; 483 return RHS; 484 } 485 } 486 } 487 488 Cache[Cond] = nullptr; 489 return nullptr; 490 } 491 492 /// Cond is a condition that occurs in L. If it is invariant in the loop, or has 493 /// an invariant piece, return the invariant along with the operator chain type. 494 /// Otherwise, return null. 495 static std::pair<Value *, OperatorChain> FindLIVLoopCondition(Value *Cond, 496 Loop *L, 497 bool &Changed) { 498 DenseMap<Value *, Value *> Cache; 499 OperatorChain OpChain = OC_OpChainNone; 500 Value *FCond = FindLIVLoopCondition(Cond, L, Changed, OpChain, Cache); 501 502 // In case we do find a LIV, it can not be obtained by walking up a mixed 503 // operator chain. 504 assert((!FCond || OpChain != OC_OpChainMixed) && 505 "Do not expect a partial LIV with mixed operator chain"); 506 return {FCond, OpChain}; 507 } 508 509 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { 510 if (skipLoop(L)) 511 return false; 512 513 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache( 514 *L->getHeader()->getParent()); 515 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 516 LPM = &LPM_Ref; 517 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 518 currentLoop = L; 519 Function *F = currentLoop->getHeader()->getParent(); 520 521 SanitizeMemory = F->hasFnAttribute(Attribute::SanitizeMemory); 522 if (SanitizeMemory) 523 computeLoopSafetyInfo(&SafetyInfo, L); 524 525 bool Changed = false; 526 do { 527 assert(currentLoop->isLCSSAForm(*DT)); 528 redoLoop = false; 529 Changed |= processCurrentLoop(); 530 } while(redoLoop); 531 532 return Changed; 533 } 534 535 // Return true if the BasicBlock BB is unreachable from the loop header. 536 // Return false, otherwise. 537 bool LoopUnswitch::isUnreachableDueToPreviousUnswitching(BasicBlock *BB) { 538 auto *Node = DT->getNode(BB)->getIDom(); 539 BasicBlock *DomBB = Node->getBlock(); 540 while (currentLoop->contains(DomBB)) { 541 BranchInst *BInst = dyn_cast<BranchInst>(DomBB->getTerminator()); 542 543 Node = DT->getNode(DomBB)->getIDom(); 544 DomBB = Node->getBlock(); 545 546 if (!BInst || !BInst->isConditional()) 547 continue; 548 549 Value *Cond = BInst->getCondition(); 550 if (!isa<ConstantInt>(Cond)) 551 continue; 552 553 BasicBlock *UnreachableSucc = 554 Cond == ConstantInt::getTrue(Cond->getContext()) 555 ? BInst->getSuccessor(1) 556 : BInst->getSuccessor(0); 557 558 if (DT->dominates(UnreachableSucc, BB)) 559 return true; 560 } 561 return false; 562 } 563 564 /// FIXME: Remove this workaround when freeze related patches are done. 565 /// LoopUnswitch and Equality propagation in GVN have discrepancy about 566 /// whether branch on undef/poison has undefine behavior. Here it is to 567 /// rule out some common cases that we found such discrepancy already 568 /// causing problems. Detail could be found in PR31652. Note if the 569 /// func returns true, it is unsafe. But if it is false, it doesn't mean 570 /// it is necessarily safe. 571 static bool EqualityPropUnSafe(Value &LoopCond) { 572 ICmpInst *CI = dyn_cast<ICmpInst>(&LoopCond); 573 if (!CI || !CI->isEquality()) 574 return false; 575 576 Value *LHS = CI->getOperand(0); 577 Value *RHS = CI->getOperand(1); 578 if (isa<UndefValue>(LHS) || isa<UndefValue>(RHS)) 579 return true; 580 581 auto hasUndefInPHI = [](PHINode &PN) { 582 for (Value *Opd : PN.incoming_values()) { 583 if (isa<UndefValue>(Opd)) 584 return true; 585 } 586 return false; 587 }; 588 PHINode *LPHI = dyn_cast<PHINode>(LHS); 589 PHINode *RPHI = dyn_cast<PHINode>(RHS); 590 if ((LPHI && hasUndefInPHI(*LPHI)) || (RPHI && hasUndefInPHI(*RPHI))) 591 return true; 592 593 auto hasUndefInSelect = [](SelectInst &SI) { 594 if (isa<UndefValue>(SI.getTrueValue()) || 595 isa<UndefValue>(SI.getFalseValue())) 596 return true; 597 return false; 598 }; 599 SelectInst *LSI = dyn_cast<SelectInst>(LHS); 600 SelectInst *RSI = dyn_cast<SelectInst>(RHS); 601 if ((LSI && hasUndefInSelect(*LSI)) || (RSI && hasUndefInSelect(*RSI))) 602 return true; 603 return false; 604 } 605 606 /// Do actual work and unswitch loop if possible and profitable. 607 bool LoopUnswitch::processCurrentLoop() { 608 bool Changed = false; 609 610 initLoopData(); 611 612 // If LoopSimplify was unable to form a preheader, don't do any unswitching. 613 if (!loopPreheader) 614 return false; 615 616 // Loops with indirectbr cannot be cloned. 617 if (!currentLoop->isSafeToClone()) 618 return false; 619 620 // Without dedicated exits, splitting the exit edge may fail. 621 if (!currentLoop->hasDedicatedExits()) 622 return false; 623 624 LLVMContext &Context = loopHeader->getContext(); 625 626 // Analyze loop cost, and stop unswitching if loop content can not be duplicated. 627 if (!BranchesInfo.countLoop( 628 currentLoop, getAnalysis<TargetTransformInfoWrapperPass>().getTTI( 629 *currentLoop->getHeader()->getParent()), 630 AC)) 631 return false; 632 633 // Try trivial unswitch first before loop over other basic blocks in the loop. 634 if (TryTrivialLoopUnswitch(Changed)) { 635 return true; 636 } 637 638 // Do not do non-trivial unswitch while optimizing for size. 639 // FIXME: Use Function::optForSize(). 640 if (OptimizeForSize || 641 loopHeader->getParent()->hasFnAttribute(Attribute::OptimizeForSize)) 642 return false; 643 644 // Run through the instructions in the loop, keeping track of three things: 645 // 646 // - That we do not unswitch loops containing convergent operations, as we 647 // might be making them control dependent on the unswitch value when they 648 // were not before. 649 // FIXME: This could be refined to only bail if the convergent operation is 650 // not already control-dependent on the unswitch value. 651 // 652 // - That basic blocks in the loop contain invokes whose predecessor edges we 653 // cannot split. 654 // 655 // - The set of guard intrinsics encountered (these are non terminator 656 // instructions that are also profitable to be unswitched). 657 658 SmallVector<IntrinsicInst *, 4> Guards; 659 660 for (const auto BB : currentLoop->blocks()) { 661 for (auto &I : *BB) { 662 auto CS = CallSite(&I); 663 if (!CS) continue; 664 if (CS.hasFnAttr(Attribute::Convergent)) 665 return false; 666 if (auto *II = dyn_cast<InvokeInst>(&I)) 667 if (!II->getUnwindDest()->canSplitPredecessors()) 668 return false; 669 if (auto *II = dyn_cast<IntrinsicInst>(&I)) 670 if (II->getIntrinsicID() == Intrinsic::experimental_guard) 671 Guards.push_back(II); 672 } 673 } 674 675 for (IntrinsicInst *Guard : Guards) { 676 Value *LoopCond = 677 FindLIVLoopCondition(Guard->getOperand(0), currentLoop, Changed).first; 678 if (LoopCond && 679 UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context))) { 680 // NB! Unswitching (if successful) could have erased some of the 681 // instructions in Guards leaving dangling pointers there. This is fine 682 // because we're returning now, and won't look at Guards again. 683 ++NumGuards; 684 return true; 685 } 686 } 687 688 // Loop over all of the basic blocks in the loop. If we find an interior 689 // block that is branching on a loop-invariant condition, we can unswitch this 690 // loop. 691 for (Loop::block_iterator I = currentLoop->block_begin(), 692 E = currentLoop->block_end(); I != E; ++I) { 693 TerminatorInst *TI = (*I)->getTerminator(); 694 695 // Unswitching on a potentially uninitialized predicate is not 696 // MSan-friendly. Limit this to the cases when the original predicate is 697 // guaranteed to execute, to avoid creating a use-of-uninitialized-value 698 // in the code that did not have one. 699 // This is a workaround for the discrepancy between LLVM IR and MSan 700 // semantics. See PR28054 for more details. 701 if (SanitizeMemory && 702 !isGuaranteedToExecute(*TI, DT, currentLoop, &SafetyInfo)) 703 continue; 704 705 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 706 // Some branches may be rendered unreachable because of previous 707 // unswitching. 708 // Unswitch only those branches that are reachable. 709 if (isUnreachableDueToPreviousUnswitching(*I)) 710 continue; 711 712 // If this isn't branching on an invariant condition, we can't unswitch 713 // it. 714 if (BI->isConditional()) { 715 // See if this, or some part of it, is loop invariant. If so, we can 716 // unswitch on it if we desire. 717 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), 718 currentLoop, Changed).first; 719 if (LoopCond && !EqualityPropUnSafe(*LoopCond) && 720 UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) { 721 ++NumBranches; 722 return true; 723 } 724 } 725 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 726 Value *SC = SI->getCondition(); 727 Value *LoopCond; 728 OperatorChain OpChain; 729 std::tie(LoopCond, OpChain) = 730 FindLIVLoopCondition(SC, currentLoop, Changed); 731 732 unsigned NumCases = SI->getNumCases(); 733 if (LoopCond && NumCases) { 734 // Find a value to unswitch on: 735 // FIXME: this should chose the most expensive case! 736 // FIXME: scan for a case with a non-critical edge? 737 Constant *UnswitchVal = nullptr; 738 // Find a case value such that at least one case value is unswitched 739 // out. 740 if (OpChain == OC_OpChainAnd) { 741 // If the chain only has ANDs and the switch has a case value of 0. 742 // Dropping in a 0 to the chain will unswitch out the 0-casevalue. 743 auto *AllZero = cast<ConstantInt>(Constant::getNullValue(SC->getType())); 744 if (BranchesInfo.isUnswitched(SI, AllZero)) 745 continue; 746 // We are unswitching 0 out. 747 UnswitchVal = AllZero; 748 } else if (OpChain == OC_OpChainOr) { 749 // If the chain only has ORs and the switch has a case value of ~0. 750 // Dropping in a ~0 to the chain will unswitch out the ~0-casevalue. 751 auto *AllOne = cast<ConstantInt>(Constant::getAllOnesValue(SC->getType())); 752 if (BranchesInfo.isUnswitched(SI, AllOne)) 753 continue; 754 // We are unswitching ~0 out. 755 UnswitchVal = AllOne; 756 } else { 757 assert(OpChain == OC_OpChainNone && 758 "Expect to unswitch on trivial chain"); 759 // Do not process same value again and again. 760 // At this point we have some cases already unswitched and 761 // some not yet unswitched. Let's find the first not yet unswitched one. 762 for (auto Case : SI->cases()) { 763 Constant *UnswitchValCandidate = Case.getCaseValue(); 764 if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) { 765 UnswitchVal = UnswitchValCandidate; 766 break; 767 } 768 } 769 } 770 771 if (!UnswitchVal) 772 continue; 773 774 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { 775 ++NumSwitches; 776 // In case of a full LIV, UnswitchVal is the value we unswitched out. 777 // In case of a partial LIV, we only unswitch when its an AND-chain 778 // or OR-chain. In both cases switch input value simplifies to 779 // UnswitchVal. 780 BranchesInfo.setUnswitched(SI, UnswitchVal); 781 return true; 782 } 783 } 784 } 785 786 // Scan the instructions to check for unswitchable values. 787 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 788 BBI != E; ++BBI) 789 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 790 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 791 currentLoop, Changed).first; 792 if (LoopCond && UnswitchIfProfitable(LoopCond, 793 ConstantInt::getTrue(Context))) { 794 ++NumSelects; 795 return true; 796 } 797 } 798 } 799 return Changed; 800 } 801 802 /// Check to see if all paths from BB exit the loop with no side effects 803 /// (including infinite loops). 804 /// 805 /// If true, we return true and set ExitBB to the block we 806 /// exit through. 807 /// 808 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, 809 BasicBlock *&ExitBB, 810 std::set<BasicBlock*> &Visited) { 811 if (!Visited.insert(BB).second) { 812 // Already visited. Without more analysis, this could indicate an infinite 813 // loop. 814 return false; 815 } 816 if (!L->contains(BB)) { 817 // Otherwise, this is a loop exit, this is fine so long as this is the 818 // first exit. 819 if (ExitBB) return false; 820 ExitBB = BB; 821 return true; 822 } 823 824 // Otherwise, this is an unvisited intra-loop node. Check all successors. 825 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { 826 // Check to see if the successor is a trivial loop exit. 827 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) 828 return false; 829 } 830 831 // Okay, everything after this looks good, check to make sure that this block 832 // doesn't include any side effects. 833 for (Instruction &I : *BB) 834 if (I.mayHaveSideEffects()) 835 return false; 836 837 return true; 838 } 839 840 /// Return true if the specified block unconditionally leads to an exit from 841 /// the specified loop, and has no side-effects in the process. If so, return 842 /// the block that is exited to, otherwise return null. 843 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { 844 std::set<BasicBlock*> Visited; 845 Visited.insert(L->getHeader()); // Branches to header make infinite loops. 846 BasicBlock *ExitBB = nullptr; 847 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) 848 return ExitBB; 849 return nullptr; 850 } 851 852 /// We have found that we can unswitch currentLoop when LoopCond == Val to 853 /// simplify the loop. If we decide that this is profitable, 854 /// unswitch the loop, reprocess the pieces, then return true. 855 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val, 856 TerminatorInst *TI) { 857 // Check to see if it would be profitable to unswitch current loop. 858 if (!BranchesInfo.CostAllowsUnswitching()) { 859 LLVM_DEBUG(dbgs() << "NOT unswitching loop %" 860 << currentLoop->getHeader()->getName() 861 << " at non-trivial condition '" << *Val 862 << "' == " << *LoopCond << "\n" 863 << ". Cost too high.\n"); 864 return false; 865 } 866 if (hasBranchDivergence && 867 getAnalysis<DivergenceAnalysis>().isDivergent(LoopCond)) { 868 LLVM_DEBUG(dbgs() << "NOT unswitching loop %" 869 << currentLoop->getHeader()->getName() 870 << " at non-trivial condition '" << *Val 871 << "' == " << *LoopCond << "\n" 872 << ". Condition is divergent.\n"); 873 return false; 874 } 875 876 UnswitchNontrivialCondition(LoopCond, Val, currentLoop, TI); 877 return true; 878 } 879 880 /// Recursively clone the specified loop and all of its children, 881 /// mapping the blocks with the specified map. 882 static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM, 883 LoopInfo *LI, LPPassManager *LPM) { 884 Loop &New = *LI->AllocateLoop(); 885 if (PL) 886 PL->addChildLoop(&New); 887 else 888 LI->addTopLevelLoop(&New); 889 LPM->addLoop(New); 890 891 // Add all of the blocks in L to the new loop. 892 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 893 I != E; ++I) 894 if (LI->getLoopFor(*I) == L) 895 New.addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI); 896 897 // Add all of the subloops to the new loop. 898 for (Loop *I : *L) 899 CloneLoop(I, &New, VM, LI, LPM); 900 901 return &New; 902 } 903 904 /// Emit a conditional branch on two values if LIC == Val, branch to TrueDst, 905 /// otherwise branch to FalseDest. Insert the code immediately before OldBranch 906 /// and remove (but not erase!) it from the function. 907 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 908 BasicBlock *TrueDest, 909 BasicBlock *FalseDest, 910 BranchInst *OldBranch, 911 TerminatorInst *TI) { 912 assert(OldBranch->isUnconditional() && "Preheader is not split correctly"); 913 assert(TrueDest != FalseDest && "Branch targets should be different"); 914 // Insert a conditional branch on LIC to the two preheaders. The original 915 // code is the true version and the new code is the false version. 916 Value *BranchVal = LIC; 917 bool Swapped = false; 918 if (!isa<ConstantInt>(Val) || 919 Val->getType() != Type::getInt1Ty(LIC->getContext())) 920 BranchVal = new ICmpInst(OldBranch, ICmpInst::ICMP_EQ, LIC, Val); 921 else if (Val != ConstantInt::getTrue(Val->getContext())) { 922 // We want to enter the new loop when the condition is true. 923 std::swap(TrueDest, FalseDest); 924 Swapped = true; 925 } 926 927 // Old branch will be removed, so save its parent and successor to update the 928 // DomTree. 929 auto *OldBranchSucc = OldBranch->getSuccessor(0); 930 auto *OldBranchParent = OldBranch->getParent(); 931 932 // Insert the new branch. 933 BranchInst *BI = 934 IRBuilder<>(OldBranch).CreateCondBr(BranchVal, TrueDest, FalseDest, TI); 935 if (Swapped) 936 BI->swapProfMetadata(); 937 938 // Remove the old branch so there is only one branch at the end. This is 939 // needed to perform DomTree's internal DFS walk on the function's CFG. 940 OldBranch->removeFromParent(); 941 942 // Inform the DT about the new branch. 943 if (DT) { 944 // First, add both successors. 945 SmallVector<DominatorTree::UpdateType, 3> Updates; 946 if (TrueDest != OldBranchSucc) 947 Updates.push_back({DominatorTree::Insert, OldBranchParent, TrueDest}); 948 if (FalseDest != OldBranchSucc) 949 Updates.push_back({DominatorTree::Insert, OldBranchParent, FalseDest}); 950 // If both of the new successors are different from the old one, inform the 951 // DT that the edge was deleted. 952 if (OldBranchSucc != TrueDest && OldBranchSucc != FalseDest) { 953 Updates.push_back({DominatorTree::Delete, OldBranchParent, OldBranchSucc}); 954 } 955 956 DT->applyUpdates(Updates); 957 } 958 959 // If either edge is critical, split it. This helps preserve LoopSimplify 960 // form for enclosing loops. 961 auto Options = CriticalEdgeSplittingOptions(DT, LI).setPreserveLCSSA(); 962 SplitCriticalEdge(BI, 0, Options); 963 SplitCriticalEdge(BI, 1, Options); 964 } 965 966 /// Given a loop that has a trivial unswitchable condition in it (a cond branch 967 /// from its header block to its latch block, where the path through the loop 968 /// that doesn't execute its body has no side-effects), unswitch it. This 969 /// doesn't involve any code duplication, just moving the conditional branch 970 /// outside of the loop and updating loop info. 971 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 972 BasicBlock *ExitBlock, 973 TerminatorInst *TI) { 974 LLVM_DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %" 975 << loopHeader->getName() << " [" << L->getBlocks().size() 976 << " blocks] in Function " 977 << L->getHeader()->getParent()->getName() 978 << " on cond: " << *Val << " == " << *Cond << "\n"); 979 // We are going to make essential changes to CFG. This may invalidate cached 980 // information for L or one of its parent loops in SCEV. 981 if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>()) 982 SEWP->getSE().forgetTopmostLoop(L); 983 984 // First step, split the preheader, so that we know that there is a safe place 985 // to insert the conditional branch. We will change loopPreheader to have a 986 // conditional branch on Cond. 987 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, DT, LI); 988 989 // Now that we have a place to insert the conditional branch, create a place 990 // to branch to: this is the exit block out of the loop that we should 991 // short-circuit to. 992 993 // Split this block now, so that the loop maintains its exit block, and so 994 // that the jump from the preheader can execute the contents of the exit block 995 // without actually branching to it (the exit block should be dominated by the 996 // loop header, not the preheader). 997 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 998 BasicBlock *NewExit = SplitBlock(ExitBlock, &ExitBlock->front(), DT, LI); 999 1000 // Okay, now we have a position to branch from and a position to branch to, 1001 // insert the new conditional branch. 1002 auto *OldBranch = dyn_cast<BranchInst>(loopPreheader->getTerminator()); 1003 assert(OldBranch && "Failed to split the preheader"); 1004 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, OldBranch, TI); 1005 LPM->deleteSimpleAnalysisValue(OldBranch, L); 1006 1007 // EmitPreheaderBranchOnCondition removed the OldBranch from the function. 1008 // Delete it, as it is no longer needed. 1009 delete OldBranch; 1010 1011 // We need to reprocess this loop, it could be unswitched again. 1012 redoLoop = true; 1013 1014 // Now that we know that the loop is never entered when this condition is a 1015 // particular value, rewrite the loop with this info. We know that this will 1016 // at least eliminate the old branch. 1017 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); 1018 ++NumTrivial; 1019 } 1020 1021 /// Check if the first non-constant condition starting from the loop header is 1022 /// a trivial unswitch condition: that is, a condition controls whether or not 1023 /// the loop does anything at all. If it is a trivial condition, unswitching 1024 /// produces no code duplications (equivalently, it produces a simpler loop and 1025 /// a new empty loop, which gets deleted). Therefore always unswitch trivial 1026 /// condition. 1027 bool LoopUnswitch::TryTrivialLoopUnswitch(bool &Changed) { 1028 BasicBlock *CurrentBB = currentLoop->getHeader(); 1029 TerminatorInst *CurrentTerm = CurrentBB->getTerminator(); 1030 LLVMContext &Context = CurrentBB->getContext(); 1031 1032 // If loop header has only one reachable successor (currently via an 1033 // unconditional branch or constant foldable conditional branch, but 1034 // should also consider adding constant foldable switch instruction in 1035 // future), we should keep looking for trivial condition candidates in 1036 // the successor as well. An alternative is to constant fold conditions 1037 // and merge successors into loop header (then we only need to check header's 1038 // terminator). The reason for not doing this in LoopUnswitch pass is that 1039 // it could potentially break LoopPassManager's invariants. Folding dead 1040 // branches could either eliminate the current loop or make other loops 1041 // unreachable. LCSSA form might also not be preserved after deleting 1042 // branches. The following code keeps traversing loop header's successors 1043 // until it finds the trivial condition candidate (condition that is not a 1044 // constant). Since unswitching generates branches with constant conditions, 1045 // this scenario could be very common in practice. 1046 SmallPtrSet<BasicBlock*, 8> Visited; 1047 1048 while (true) { 1049 // If we exit loop or reach a previous visited block, then 1050 // we can not reach any trivial condition candidates (unfoldable 1051 // branch instructions or switch instructions) and no unswitch 1052 // can happen. Exit and return false. 1053 if (!currentLoop->contains(CurrentBB) || !Visited.insert(CurrentBB).second) 1054 return false; 1055 1056 // Check if this loop will execute any side-effecting instructions (e.g. 1057 // stores, calls, volatile loads) in the part of the loop that the code 1058 // *would* execute. Check the header first. 1059 for (Instruction &I : *CurrentBB) 1060 if (I.mayHaveSideEffects()) 1061 return false; 1062 1063 if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) { 1064 if (BI->isUnconditional()) { 1065 CurrentBB = BI->getSuccessor(0); 1066 } else if (BI->getCondition() == ConstantInt::getTrue(Context)) { 1067 CurrentBB = BI->getSuccessor(0); 1068 } else if (BI->getCondition() == ConstantInt::getFalse(Context)) { 1069 CurrentBB = BI->getSuccessor(1); 1070 } else { 1071 // Found a trivial condition candidate: non-foldable conditional branch. 1072 break; 1073 } 1074 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurrentTerm)) { 1075 // At this point, any constant-foldable instructions should have probably 1076 // been folded. 1077 ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition()); 1078 if (!Cond) 1079 break; 1080 // Find the target block we are definitely going to. 1081 CurrentBB = SI->findCaseValue(Cond)->getCaseSuccessor(); 1082 } else { 1083 // We do not understand these terminator instructions. 1084 break; 1085 } 1086 1087 CurrentTerm = CurrentBB->getTerminator(); 1088 } 1089 1090 // CondVal is the condition that controls the trivial condition. 1091 // LoopExitBB is the BasicBlock that loop exits when meets trivial condition. 1092 Constant *CondVal = nullptr; 1093 BasicBlock *LoopExitBB = nullptr; 1094 1095 if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) { 1096 // If this isn't branching on an invariant condition, we can't unswitch it. 1097 if (!BI->isConditional()) 1098 return false; 1099 1100 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), 1101 currentLoop, Changed).first; 1102 1103 // Unswitch only if the trivial condition itself is an LIV (not 1104 // partial LIV which could occur in and/or) 1105 if (!LoopCond || LoopCond != BI->getCondition()) 1106 return false; 1107 1108 // Check to see if a successor of the branch is guaranteed to 1109 // exit through a unique exit block without having any 1110 // side-effects. If so, determine the value of Cond that causes 1111 // it to do this. 1112 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 1113 BI->getSuccessor(0)))) { 1114 CondVal = ConstantInt::getTrue(Context); 1115 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 1116 BI->getSuccessor(1)))) { 1117 CondVal = ConstantInt::getFalse(Context); 1118 } 1119 1120 // If we didn't find a single unique LoopExit block, or if the loop exit 1121 // block contains phi nodes, this isn't trivial. 1122 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) 1123 return false; // Can't handle this. 1124 1125 if (EqualityPropUnSafe(*LoopCond)) 1126 return false; 1127 1128 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB, 1129 CurrentTerm); 1130 ++NumBranches; 1131 return true; 1132 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurrentTerm)) { 1133 // If this isn't switching on an invariant condition, we can't unswitch it. 1134 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 1135 currentLoop, Changed).first; 1136 1137 // Unswitch only if the trivial condition itself is an LIV (not 1138 // partial LIV which could occur in and/or) 1139 if (!LoopCond || LoopCond != SI->getCondition()) 1140 return false; 1141 1142 // Check to see if a successor of the switch is guaranteed to go to the 1143 // latch block or exit through a one exit block without having any 1144 // side-effects. If so, determine the value of Cond that causes it to do 1145 // this. 1146 // Note that we can't trivially unswitch on the default case or 1147 // on already unswitched cases. 1148 for (auto Case : SI->cases()) { 1149 BasicBlock *LoopExitCandidate; 1150 if ((LoopExitCandidate = 1151 isTrivialLoopExitBlock(currentLoop, Case.getCaseSuccessor()))) { 1152 // Okay, we found a trivial case, remember the value that is trivial. 1153 ConstantInt *CaseVal = Case.getCaseValue(); 1154 1155 // Check that it was not unswitched before, since already unswitched 1156 // trivial vals are looks trivial too. 1157 if (BranchesInfo.isUnswitched(SI, CaseVal)) 1158 continue; 1159 LoopExitBB = LoopExitCandidate; 1160 CondVal = CaseVal; 1161 break; 1162 } 1163 } 1164 1165 // If we didn't find a single unique LoopExit block, or if the loop exit 1166 // block contains phi nodes, this isn't trivial. 1167 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) 1168 return false; // Can't handle this. 1169 1170 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB, 1171 nullptr); 1172 1173 // We are only unswitching full LIV. 1174 BranchesInfo.setUnswitched(SI, CondVal); 1175 ++NumSwitches; 1176 return true; 1177 } 1178 return false; 1179 } 1180 1181 /// Split all of the edges from inside the loop to their exit blocks. 1182 /// Update the appropriate Phi nodes as we do so. 1183 void LoopUnswitch::SplitExitEdges(Loop *L, 1184 const SmallVectorImpl<BasicBlock *> &ExitBlocks){ 1185 1186 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 1187 BasicBlock *ExitBlock = ExitBlocks[i]; 1188 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock), 1189 pred_end(ExitBlock)); 1190 1191 // Although SplitBlockPredecessors doesn't preserve loop-simplify in 1192 // general, if we call it on all predecessors of all exits then it does. 1193 SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", DT, LI, 1194 /*PreserveLCSSA*/ true); 1195 } 1196 } 1197 1198 /// We determined that the loop is profitable to unswitch when LIC equal Val. 1199 /// Split it into loop versions and test the condition outside of either loop. 1200 /// Return the loops created as Out1/Out2. 1201 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, 1202 Loop *L, TerminatorInst *TI) { 1203 Function *F = loopHeader->getParent(); 1204 LLVM_DEBUG(dbgs() << "loop-unswitch: Unswitching loop %" 1205 << loopHeader->getName() << " [" << L->getBlocks().size() 1206 << " blocks] in Function " << F->getName() << " when '" 1207 << *Val << "' == " << *LIC << "\n"); 1208 1209 // We are going to make essential changes to CFG. This may invalidate cached 1210 // information for L or one of its parent loops in SCEV. 1211 if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>()) 1212 SEWP->getSE().forgetTopmostLoop(L); 1213 1214 LoopBlocks.clear(); 1215 NewBlocks.clear(); 1216 1217 // First step, split the preheader and exit blocks, and add these blocks to 1218 // the LoopBlocks list. 1219 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, DT, LI); 1220 LoopBlocks.push_back(NewPreheader); 1221 1222 // We want the loop to come after the preheader, but before the exit blocks. 1223 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 1224 1225 SmallVector<BasicBlock*, 8> ExitBlocks; 1226 L->getUniqueExitBlocks(ExitBlocks); 1227 1228 // Split all of the edges from inside the loop to their exit blocks. Update 1229 // the appropriate Phi nodes as we do so. 1230 SplitExitEdges(L, ExitBlocks); 1231 1232 // The exit blocks may have been changed due to edge splitting, recompute. 1233 ExitBlocks.clear(); 1234 L->getUniqueExitBlocks(ExitBlocks); 1235 1236 // Add exit blocks to the loop blocks. 1237 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 1238 1239 // Next step, clone all of the basic blocks that make up the loop (including 1240 // the loop preheader and exit blocks), keeping track of the mapping between 1241 // the instructions and blocks. 1242 NewBlocks.reserve(LoopBlocks.size()); 1243 ValueToValueMapTy VMap; 1244 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 1245 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F); 1246 1247 NewBlocks.push_back(NewBB); 1248 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping. 1249 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L); 1250 } 1251 1252 // Splice the newly inserted blocks into the function right before the 1253 // original preheader. 1254 F->getBasicBlockList().splice(NewPreheader->getIterator(), 1255 F->getBasicBlockList(), 1256 NewBlocks[0]->getIterator(), F->end()); 1257 1258 // Now we create the new Loop object for the versioned loop. 1259 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM); 1260 1261 // Recalculate unswitching quota, inherit simplified switches info for NewBB, 1262 // Probably clone more loop-unswitch related loop properties. 1263 BranchesInfo.cloneData(NewLoop, L, VMap); 1264 1265 Loop *ParentLoop = L->getParentLoop(); 1266 if (ParentLoop) { 1267 // Make sure to add the cloned preheader and exit blocks to the parent loop 1268 // as well. 1269 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI); 1270 } 1271 1272 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 1273 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]); 1274 // The new exit block should be in the same loop as the old one. 1275 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) 1276 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI); 1277 1278 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 1279 "Exit block should have been split to have one successor!"); 1280 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 1281 1282 // If the successor of the exit block had PHI nodes, add an entry for 1283 // NewExit. 1284 for (PHINode &PN : ExitSucc->phis()) { 1285 Value *V = PN.getIncomingValueForBlock(ExitBlocks[i]); 1286 ValueToValueMapTy::iterator It = VMap.find(V); 1287 if (It != VMap.end()) V = It->second; 1288 PN.addIncoming(V, NewExit); 1289 } 1290 1291 if (LandingPadInst *LPad = NewExit->getLandingPadInst()) { 1292 PHINode *PN = PHINode::Create(LPad->getType(), 0, "", 1293 &*ExitSucc->getFirstInsertionPt()); 1294 1295 for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc); 1296 I != E; ++I) { 1297 BasicBlock *BB = *I; 1298 LandingPadInst *LPI = BB->getLandingPadInst(); 1299 LPI->replaceAllUsesWith(PN); 1300 PN->addIncoming(LPI, BB); 1301 } 1302 } 1303 } 1304 1305 // Rewrite the code to refer to itself. 1306 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) { 1307 for (Instruction &I : *NewBlocks[i]) { 1308 RemapInstruction(&I, VMap, 1309 RF_NoModuleLevelChanges | RF_IgnoreMissingLocals); 1310 if (auto *II = dyn_cast<IntrinsicInst>(&I)) 1311 if (II->getIntrinsicID() == Intrinsic::assume) 1312 AC->registerAssumption(II); 1313 } 1314 } 1315 1316 // Rewrite the original preheader to select between versions of the loop. 1317 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator()); 1318 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 1319 "Preheader splitting did not work correctly!"); 1320 1321 // Emit the new branch that selects between the two versions of this loop. 1322 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR, 1323 TI); 1324 LPM->deleteSimpleAnalysisValue(OldBR, L); 1325 1326 // The OldBr was replaced by a new one and removed (but not erased) by 1327 // EmitPreheaderBranchOnCondition. It is no longer needed, so delete it. 1328 delete OldBR; 1329 1330 LoopProcessWorklist.push_back(NewLoop); 1331 redoLoop = true; 1332 1333 // Keep a WeakTrackingVH holding onto LIC. If the first call to 1334 // RewriteLoopBody 1335 // deletes the instruction (for example by simplifying a PHI that feeds into 1336 // the condition that we're unswitching on), we don't rewrite the second 1337 // iteration. 1338 WeakTrackingVH LICHandle(LIC); 1339 1340 // Now we rewrite the original code to know that the condition is true and the 1341 // new code to know that the condition is false. 1342 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false); 1343 1344 // It's possible that simplifying one loop could cause the other to be 1345 // changed to another value or a constant. If its a constant, don't simplify 1346 // it. 1347 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop && 1348 LICHandle && !isa<Constant>(LICHandle)) 1349 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true); 1350 } 1351 1352 /// Remove all instances of I from the worklist vector specified. 1353 static void RemoveFromWorklist(Instruction *I, 1354 std::vector<Instruction*> &Worklist) { 1355 1356 Worklist.erase(std::remove(Worklist.begin(), Worklist.end(), I), 1357 Worklist.end()); 1358 } 1359 1360 /// When we find that I really equals V, remove I from the 1361 /// program, replacing all uses with V and update the worklist. 1362 static void ReplaceUsesOfWith(Instruction *I, Value *V, 1363 std::vector<Instruction*> &Worklist, 1364 Loop *L, LPPassManager *LPM) { 1365 LLVM_DEBUG(dbgs() << "Replace with '" << *V << "': " << *I << "\n"); 1366 1367 // Add uses to the worklist, which may be dead now. 1368 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 1369 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 1370 Worklist.push_back(Use); 1371 1372 // Add users to the worklist which may be simplified now. 1373 for (User *U : I->users()) 1374 Worklist.push_back(cast<Instruction>(U)); 1375 LPM->deleteSimpleAnalysisValue(I, L); 1376 RemoveFromWorklist(I, Worklist); 1377 I->replaceAllUsesWith(V); 1378 if (!I->mayHaveSideEffects()) 1379 I->eraseFromParent(); 1380 ++NumSimplify; 1381 } 1382 1383 /// We know either that the value LIC has the value specified by Val in the 1384 /// specified loop, or we know it does NOT have that value. 1385 /// Rewrite any uses of LIC or of properties correlated to it. 1386 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 1387 Constant *Val, 1388 bool IsEqual) { 1389 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 1390 1391 // FIXME: Support correlated properties, like: 1392 // for (...) 1393 // if (li1 < li2) 1394 // ... 1395 // if (li1 > li2) 1396 // ... 1397 1398 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 1399 // selects, switches. 1400 std::vector<Instruction*> Worklist; 1401 LLVMContext &Context = Val->getContext(); 1402 1403 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC 1404 // in the loop with the appropriate one directly. 1405 if (IsEqual || (isa<ConstantInt>(Val) && 1406 Val->getType()->isIntegerTy(1))) { 1407 Value *Replacement; 1408 if (IsEqual) 1409 Replacement = Val; 1410 else 1411 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()), 1412 !cast<ConstantInt>(Val)->getZExtValue()); 1413 1414 for (User *U : LIC->users()) { 1415 Instruction *UI = dyn_cast<Instruction>(U); 1416 if (!UI || !L->contains(UI)) 1417 continue; 1418 Worklist.push_back(UI); 1419 } 1420 1421 for (Instruction *UI : Worklist) 1422 UI->replaceUsesOfWith(LIC, Replacement); 1423 1424 SimplifyCode(Worklist, L); 1425 return; 1426 } 1427 1428 // Otherwise, we don't know the precise value of LIC, but we do know that it 1429 // is certainly NOT "Val". As such, simplify any uses in the loop that we 1430 // can. This case occurs when we unswitch switch statements. 1431 for (User *U : LIC->users()) { 1432 Instruction *UI = dyn_cast<Instruction>(U); 1433 if (!UI || !L->contains(UI)) 1434 continue; 1435 1436 // At this point, we know LIC is definitely not Val. Try to use some simple 1437 // logic to simplify the user w.r.t. to the context. 1438 if (Value *Replacement = SimplifyInstructionWithNotEqual(UI, LIC, Val)) { 1439 if (LI->replacementPreservesLCSSAForm(UI, Replacement)) { 1440 // This in-loop instruction has been simplified w.r.t. its context, 1441 // i.e. LIC != Val, make sure we propagate its replacement value to 1442 // all its users. 1443 // 1444 // We can not yet delete UI, the LIC user, yet, because that would invalidate 1445 // the LIC->users() iterator !. However, we can make this instruction 1446 // dead by replacing all its users and push it onto the worklist so that 1447 // it can be properly deleted and its operands simplified. 1448 UI->replaceAllUsesWith(Replacement); 1449 } 1450 } 1451 1452 // This is a LIC user, push it into the worklist so that SimplifyCode can 1453 // attempt to simplify it. 1454 Worklist.push_back(UI); 1455 1456 // If we know that LIC is not Val, use this info to simplify code. 1457 SwitchInst *SI = dyn_cast<SwitchInst>(UI); 1458 if (!SI || !isa<ConstantInt>(Val)) continue; 1459 1460 // NOTE: if a case value for the switch is unswitched out, we record it 1461 // after the unswitch finishes. We can not record it here as the switch 1462 // is not a direct user of the partial LIV. 1463 SwitchInst::CaseHandle DeadCase = 1464 *SI->findCaseValue(cast<ConstantInt>(Val)); 1465 // Default case is live for multiple values. 1466 if (DeadCase == *SI->case_default()) 1467 continue; 1468 1469 // Found a dead case value. Don't remove PHI nodes in the 1470 // successor if they become single-entry, those PHI nodes may 1471 // be in the Users list. 1472 1473 BasicBlock *Switch = SI->getParent(); 1474 BasicBlock *SISucc = DeadCase.getCaseSuccessor(); 1475 BasicBlock *Latch = L->getLoopLatch(); 1476 1477 if (!SI->findCaseDest(SISucc)) continue; // Edge is critical. 1478 // If the DeadCase successor dominates the loop latch, then the 1479 // transformation isn't safe since it will delete the sole predecessor edge 1480 // to the latch. 1481 if (Latch && DT->dominates(SISucc, Latch)) 1482 continue; 1483 1484 // FIXME: This is a hack. We need to keep the successor around 1485 // and hooked up so as to preserve the loop structure, because 1486 // trying to update it is complicated. So instead we preserve the 1487 // loop structure and put the block on a dead code path. 1488 SplitEdge(Switch, SISucc, DT, LI); 1489 // Compute the successors instead of relying on the return value 1490 // of SplitEdge, since it may have split the switch successor 1491 // after PHI nodes. 1492 BasicBlock *NewSISucc = DeadCase.getCaseSuccessor(); 1493 BasicBlock *OldSISucc = *succ_begin(NewSISucc); 1494 // Create an "unreachable" destination. 1495 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable", 1496 Switch->getParent(), 1497 OldSISucc); 1498 new UnreachableInst(Context, Abort); 1499 // Force the new case destination to branch to the "unreachable" 1500 // block while maintaining a (dead) CFG edge to the old block. 1501 NewSISucc->getTerminator()->eraseFromParent(); 1502 BranchInst::Create(Abort, OldSISucc, 1503 ConstantInt::getTrue(Context), NewSISucc); 1504 // Release the PHI operands for this edge. 1505 for (PHINode &PN : NewSISucc->phis()) 1506 PN.setIncomingValue(PN.getBasicBlockIndex(Switch), 1507 UndefValue::get(PN.getType())); 1508 // Tell the domtree about the new block. We don't fully update the 1509 // domtree here -- instead we force it to do a full recomputation 1510 // after the pass is complete -- but we do need to inform it of 1511 // new blocks. 1512 DT->addNewBlock(Abort, NewSISucc); 1513 } 1514 1515 SimplifyCode(Worklist, L); 1516 } 1517 1518 /// Now that we have simplified some instructions in the loop, walk over it and 1519 /// constant prop, dce, and fold control flow where possible. Note that this is 1520 /// effectively a very simple loop-structure-aware optimizer. During processing 1521 /// of this loop, L could very well be deleted, so it must not be used. 1522 /// 1523 /// FIXME: When the loop optimizer is more mature, separate this out to a new 1524 /// pass. 1525 /// 1526 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) { 1527 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); 1528 while (!Worklist.empty()) { 1529 Instruction *I = Worklist.back(); 1530 Worklist.pop_back(); 1531 1532 // Simple DCE. 1533 if (isInstructionTriviallyDead(I)) { 1534 LLVM_DEBUG(dbgs() << "Remove dead instruction '" << *I << "\n"); 1535 1536 // Add uses to the worklist, which may be dead now. 1537 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 1538 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 1539 Worklist.push_back(Use); 1540 LPM->deleteSimpleAnalysisValue(I, L); 1541 RemoveFromWorklist(I, Worklist); 1542 I->eraseFromParent(); 1543 ++NumSimplify; 1544 continue; 1545 } 1546 1547 // See if instruction simplification can hack this up. This is common for 1548 // things like "select false, X, Y" after unswitching made the condition be 1549 // 'false'. TODO: update the domtree properly so we can pass it here. 1550 if (Value *V = SimplifyInstruction(I, DL)) 1551 if (LI->replacementPreservesLCSSAForm(I, V)) { 1552 ReplaceUsesOfWith(I, V, Worklist, L, LPM); 1553 continue; 1554 } 1555 1556 // Special case hacks that appear commonly in unswitched code. 1557 if (BranchInst *BI = dyn_cast<BranchInst>(I)) { 1558 if (BI->isUnconditional()) { 1559 // If BI's parent is the only pred of the successor, fold the two blocks 1560 // together. 1561 BasicBlock *Pred = BI->getParent(); 1562 BasicBlock *Succ = BI->getSuccessor(0); 1563 BasicBlock *SinglePred = Succ->getSinglePredecessor(); 1564 if (!SinglePred) continue; // Nothing to do. 1565 assert(SinglePred == Pred && "CFG broken"); 1566 1567 LLVM_DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- " 1568 << Succ->getName() << "\n"); 1569 1570 // Resolve any single entry PHI nodes in Succ. 1571 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) 1572 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM); 1573 1574 // If Succ has any successors with PHI nodes, update them to have 1575 // entries coming from Pred instead of Succ. 1576 Succ->replaceAllUsesWith(Pred); 1577 1578 // Move all of the successor contents from Succ to Pred. 1579 Pred->getInstList().splice(BI->getIterator(), Succ->getInstList(), 1580 Succ->begin(), Succ->end()); 1581 LPM->deleteSimpleAnalysisValue(BI, L); 1582 RemoveFromWorklist(BI, Worklist); 1583 BI->eraseFromParent(); 1584 1585 // Remove Succ from the loop tree. 1586 LI->removeBlock(Succ); 1587 LPM->deleteSimpleAnalysisValue(Succ, L); 1588 Succ->eraseFromParent(); 1589 ++NumSimplify; 1590 continue; 1591 } 1592 1593 continue; 1594 } 1595 } 1596 } 1597 1598 /// Simple simplifications we can do given the information that Cond is 1599 /// definitely not equal to Val. 1600 Value *LoopUnswitch::SimplifyInstructionWithNotEqual(Instruction *Inst, 1601 Value *Invariant, 1602 Constant *Val) { 1603 // icmp eq cond, val -> false 1604 ICmpInst *CI = dyn_cast<ICmpInst>(Inst); 1605 if (CI && CI->isEquality()) { 1606 Value *Op0 = CI->getOperand(0); 1607 Value *Op1 = CI->getOperand(1); 1608 if ((Op0 == Invariant && Op1 == Val) || (Op0 == Val && Op1 == Invariant)) { 1609 LLVMContext &Ctx = Inst->getContext(); 1610 if (CI->getPredicate() == CmpInst::ICMP_EQ) 1611 return ConstantInt::getFalse(Ctx); 1612 else 1613 return ConstantInt::getTrue(Ctx); 1614 } 1615 } 1616 1617 // FIXME: there may be other opportunities, e.g. comparison with floating 1618 // point, or Invariant - Val != 0, etc. 1619 return nullptr; 1620 } 1621